Ionizing radiation is measured by gamma cameras or similar devices comprising semiconductor sensitive elements in which the radiation is converted into induced electric charges, represented by electrons or holes in the semiconductor material. Polarization electrodes are added to attract these charges, and measurements comprise the recording of certain parameters of the pulsed signals produced by the electrodes. These pulsed signals result from the movement and possible collection of charge carriers by an electrode. The movement of a charge carrier relative to an electrode may induce a charge in the latter, without said charge carrier necessarily being collected by this electrode. This is the case for example when a charge carrier moves in the vicinity of an electrode insulated from the semiconductor material by an insulating layer. With collector electrodes, it is possible to measure collected charges and induced charges.
According to the prior art, the interactions of electromagnetic or ionizing particles in a semiconductor material generate electric signals at the electrodes connected to the detector material. By connected is meant a direct link, the electrode being in contact with the semiconductor material, or a capacitive link of the electrode then being separated from the semiconductor medium by a thin layer of dielectric material. The signals measured by the electrodes are then processed and recorded over a certain period, called an acquisition period, to form a measurement spectrum. Single-dimension spectra are frequently used, these spectra corresponding to a histogram of the amplitude of the charges collected by one or more collector electrodes over a determined period of time.
However, measurement suffers from imperfections due in particular to the imperfect transport properties of the electrons and holes in the semiconductor material. The measurement of an event therefore depends inter alia on the pathway of the generated charges and on the site from which the event derives. It has already been envisaged to correct biparametric spectra to take these disparities into account and obtain a more accurate histogram of events. The correction methods may be based on correlations between the amplitude and duration of a pulse to correct their characteristic parameters in accordance with certain criteria.
These methods use biparametric spectra according to which each point in the histogram corresponds to an interaction, or event, classified for example according to the amplitude of a pulse collected by an electrode, and to the duration of said pulse. A colour code is used to identify the number of occurrences at the different points of the histogram during the acquisition period. The utility, the construction and the processing of said biparametric spectra are described for example in U.S. Pat. No. 5,854,489, EP 0 703 751, 1 058 128, 1 004 040 and 1 598 680.
Patent EP 1 037 070 describes how to use this biparametric spectrum: calibration of the detector is used according to radiation of given energy to obtain a scalar which corresponds to a number of events in a given energy window.
Yet another cause of imperfection is the sharing of charges when one of the electrodes, generally the anode, is segmented into elementary electrodes each assigned to a different detection, which is very frequently chosen when it is desired to have an additional indication on the location of the charges, and hence on the origin of the radiation: the cloud of charges produced by a particle is often spread quite broadly to influence several of the elementary electrodes which each measure a fraction of the signal produced by the event; the energy resolution of the measurements is reduced and some ionizing particles escape detection. One mode to detect and correct these errors is given in EP 1 739 456.
Other methods make particular use of the measurements to fine-tune locating of the radiation source. Mention can be made of U.S. Pat. Nos. 6,002,741, 6,169,287, the article by Warburton “An approach to sub-pixel spatial resolution in room temperature x-ray detector arrays with good energy resolution”, Materials Research Society Symposium proceedings, vol. 487, 1997, pp. 531-536 and the article by Jaecheon Kim et al., “Three-dimensional signal correction on UltraPeRL CZT detectors”, Nuclear Science Symposium Conference Record, 2007. NSS'07.IEEE, pp. 1289-1293. However, these methods are also insufficient, notably because in general they are dedicated to the correction of a single cause of perturbation and are therefore only suitable for certain measurement situations, or because they give incomplete localization.